I'm going to start a library in C# for Navigational Meshes. I would like to use it in both WPF/Winforms and in XNA. Can I create a windows class library and still use it in XNA?
In short, yes. They're both just class libraries, the different templates just helps adding some default references.
Also see this Stackoverflow question and follow the advice in creating both types and running the resulting project files through your favorite diff tool for the details ^^